Pressure, flow rates, and temperatures are parameters that are routinely measured in medical research and clinical procedures. Examples are the measurement of pressure in the bladder, in the urethra, and in the coronary arteries.
Various types of catheters have been devised for these purposes, but in general they can be classified as very delicate, very expensive, subject to substantial errors, or all three. For example, some catheters are fluid filled and measure pressures conveyed by the catheter to the outside of the body. Such catheters are subject to error caused by changes in patient position. Then the fluid column and therefore the readout pressure varies unless the patient is required to remain in one position, which is not always possible or desirable. In addition, the viscosity of the fluid introduces errors which are pressure and temperature related. Even more, because they are temperature sensitive, errors in readings can result because the room temperature can vary during a test. Also because of air conditioning, the room temperature is lower than the body temperature. For this reason, electronic compensation has to be applied.
For these and other reasons other types of catheters have also been developed especially for cardiological procedures where measurements can be very critical.
One improved type of catheter is commercially known as the "Microtip" or miniature strain gage type catheter. These catheters are available in single sensor or multi sensor configurations. They utilize small microchips which are mounted on one side of the catheter body.
The strain gage catheter has major advantages over the use of pressure transducers which convey fluids externally for measuring pressure and flow. This is because the measurement is taken within the body cavity itself. This avoids some of the problems stated above.
However, this type introduces a group of new problems. One is the "rotational effect". In certain body cavities, for example urodynamic pressure measurements in the urethra, tissue pressure can vary significantly from one side of the cavity to the other, making it difficult to measure the genuine overall pressure in the zone being studied. Similarly, arterial pressure measuring can also suffer from the same inaccuracies, depending on the size and location of the vessel.
In addition to the other problems, one of the drawbacks of this class of catheter is its cost and extreme fragility. These cost may hundreds of dollars each and are readily damaged, especially during cleaning, sterilization and patient use.
Optical type sensors have been developed, in which light reflected from a deformable diaphram is sensed to measure the pressure. As to such sensors, their developments have been in the direction of requiring that the sensor be placed at the tip of the catheter. They cannot provide for sensors along the side of the device.
It is an object of this invention to provide a catheter which measures the conditions inside the body without requiring conveying of fluids to the outside of the body, which catheter is rugged, which is not sensitive to location around its periphery, and which can provide for a multiplicity of such sensors along the length of the catheter.